A South Korean Internet portal company filed an antitrust lawsuit Monday against Microsoft, alleging the U.S. computer software giant violated trade regulations by tying instant messenger software to its Windows operating system.

Daum Communications said it was seeking 10 billion won (or $8.7 million) in damages it claimed resulted from Microsoft's Windows XP, which includes the instant messaging system MSN Messenger.

Daum, which controls about 10 percent of Korea's instant messaging market, filed the lawsuit in a Seoul district court.

"Such anticompetitive behavior has resulted in massive financial losses to Daum," the company said.

Microsoft, based in Redmond, Wash., denied the accusation Monday.

"Microsoft has competed fairly, consistent with Korean competition law," spokesman Min Seog-seo said. "There is robust competition in the Korean instant messenger market, and that competition continues to be a good thing for Korean consumers."

Daum first filed a complaint against Microsoft with the Fair Trade Commission in 2001. Daum officials said Monday the commission is still investigating that complaint.